This invention relates to surgical implants that are designed to replace meniscal tissue and cartilage in a mammalian joint, such as a knee joint and methods to implant the same. While a knee is the primary joint of concern, the invention applies to other body joints as the hip, shoulder, elbow, temporomandibular, sternoclavicular, zygapophyseal, and wrist.
Compared to the hip the knee has a much greater dependence on passive soft tissues (menisci, ligaments, and the joint capsule) for stability and function. Although the mechanics of the two joints are different, known hip and knee implants are very similar in design, both consisting of a semi-rigid on rigid (polyethylene on CoCr) bearing surface. In many prosthetic knee implants, function and mobility are impaired because rigid structures are used to replace the natural soft tissues.
Normal anatomical knees have two pliable, mobile menisci that function to absorb shock, distribute stress, increase joint congruity, increase contact area, guide arthrokinematics, help lubrication by maintaining a fluid-film bearing surface, and provide proprioceptive input, i.e., nerve impulse via its attachment to the joint capsule. Even under physiologic loading a natural knee with natural menisci will primarily distribute stresses through a fluid film, only 10% of a load is transmitted via a solid on solid contact. Due to the fluid film bearing surface contact wear is greatly reduced. In simple terms the menisci function to reduce joint stresses, decrease wear, and help guide normal kinematics. Without menisci, peak contact stresses in the knee increase by 235% or more and degenerative changes start to progress rapidly. At 0°, 30°, and 60° of flexion, natural knees with intact menisci have approximately 6 to 8 times the contact area of typical prosthetic knee implants many of which have a similar geometry to that of a natural knee without menisci.
Typical existing knee replacements lack the functional features normally provided by the menisci and the common polyethylene on metal such as cobalt chrome (CoCr) bearing interface lacks the wear-reducing fluid film bearing surface. By adding a well-designed meniscal substitute, many shortcomings of existing knee replacements can be addressed. In theory, prosthetic menisci could have the same impact on a prosthetic knee as natural menisci do for natural knees.
The prosthetic knee meniscus of the present invention has at least one and preferably two compliant prosthetic menisci (medial and lateral in the knee) that are attached to the joint capsule and meniscal horns in a similar fashion to the way a natural meniscus is attached to a natural knee. Like a natural meniscus, the meniscal knee implant of the present invention will be able to pivot and glide on a prosthetic tibial plateau. Arthrokinematic constraint comes from the meniscal attachments and will gently guide movements, providing a highly mobile but stable joint. Also through its attachments, the Anatomical Meniscal-Bearing Knee will provide proprioceptive input, giving the central nervous system feedback for refined motor control.
A preferred material for the meniscal implant of the present invention is polyurethane. Polyurethane can be made flexible so it can conform to the femoral and tibial components, thus giving the knee a large contact area throughout the entire range of motion. Such a polyurethane is described in U.S. Pat. No. 5,879,387. Alternatively, a hydrogel such as a poly(vinyl) alcohol can be used as a prosthetic meniscal implant. Such a hydrogel can be cross-linked to increase its strength and wear properties. Like cartilage, it imbibes aqueous fluids and generates a fluid-film bearing surface.
The flexible, pliable, gel-like nature of a synthetic hydrogel (when saturated with water) arises mainly from crosslinking attachments between non-parallel fibers in the gel. Depending on the specific polymeric structure that has been chosen, these crosslinking attachments between the long “backbone” chains in a polymer can be formed by covalent bonding, by hydrogen bonding or similar ionic attraction, or by entangling chains that have relatively long and/or “grabby” side-chains.
Regardless of which type of bonding or entangling method is used to bind the backbone chains together to form a hydrogel, the “coupling” points between molecular chains can usually be flexed, rotated, and stretched.
In addition, it should be recognized that the back-bone chains in hydrogel polymers are not straight; instead, because of various aspects of interatomic bonds, they are somewhat kinked, and can be stretched, in an elastic and springy manner, without breaking the bonds.
In a typical hydrogel, the fibers usually take up less than about 10% of the volume; indeed, many hydrogels contain less than 2% fiber volume, while interstitial spaces (i.e., the unoccupied spaces nestled among the three-dimensional network of fibers, which become filled with water when the gel is hydrated) usually make up at least 90 to 95% of the total volume. Accordingly, since the “coupling” point between any two polymeric backbone chains can be rotated and flexed, and since any polymeric backbone molecule can be stretched without breaking it, a supple and resilient gel-like mechanical structure results when a synthetic hydrogel polymer is hydrated.
Various methods are known for creating conventional polymeric hydrogels. A number of such methods involve mixing together and reacting precursor materials (monomers, etc.) while they are suspended in water or other solvent. This step (i.e., reacting two or more monomers while they are suspended in a solvent) gives a desired density and three-dimensional structure to the resulting polymerized strands or fibers. The resulting material is then frozen, to preserve the desired three-dimensional structure of the fibers. The ice (or other frozen solvent) is then vaporized and removed, without going through a liquid stage, by a sublimizing process (also called lyophilizing), using high vacuum and low temperature. After the solvent has been removed, any final steps (such as a final crosslinking reaction and/or rinsing or washing steps, to remove any unreacted monomers, crosslinking agents, quenching agents, etc.) are carried out. The polymer is then gradually warmed up to room temperature, and it is subsequently saturated with water, to form a completed hydrogel.
In the past, effort mainly has been placed on the development of meniscal replacement. In the attempt to repair or replace torn menisci, allographs, xenographs, and autographs have been implanted for over 20 years. Current focus has been on the development of collagen-matrix meniscal implants. However, these implants do not reproduce the mechanical properties of a normal meniscus.
As used herein, all references to “implants” or “implantation” (and all terms such as surgery, surgical, operation, etc.) refer to surgical or arthroscopic implantation of a reinforced hydrogel device, as disclosed herein, into a mammalian body or limb, such as in a human patient. Arthroscopic methods are regarded herein as a subset of surgical methods, and any reference to surgery, surgical, etc., includes arthroscopic methods and devices. The term “minimally invasive” is also used occasionally herein, even though it is imprecise; one should assume that any surgical operation will be done in a manner that is minimally invasive, in view of the needs of the patient and the goals of the surgeon.
Meniscal Tissues in Knees—Each knee joint of a human contains a “medial” meniscus, and a “lateral” meniscus. The lateral meniscus is located on the outer side of the leg, directly above the location where the upper end of the fibula bone is coupled to the tibia (“shinbone”). The medial meniscus is located on the inner side of the leg.
Each meniscus (also referred to, especially in older texts, as a “semilunar fibrocartilage”) has a wedged shape, somewhat comparable to a segment from an orange or other citric fruit, but with a substantially larger curvature and “arc.” The thickest region is around the periphery (which can also be called the circumference, the rim, and similar terms). When implanted into a knee, this peripheral rim normally will be anchored to the surrounding wall of a fibrous “capsule” which encloses the knee joint and holds in the synovial fluid, which lubricates the cartilage surfaces in the knee. The two ends of each semi-circular wedge are coupled, via thickened collagen structures called horns to the “spine” protrusions in the center of the tibial plateau.
The inner edge of a meniscus is the thinnest portion of the wedge; this edge can also be called the apex, the margin, and similar terms. It is not anchored; instead, as the person walks or runs, each meniscus in a knee is somewhat free to move, as it is squeezed between the tibial plateau (beneath it) and a femoral runner or condyle (above it). The bottom surface of each meniscus is relatively flat, so it can ride in a relatively stable manner on top of the tibial plateau. The top surface is concave, so it can provide better, more closely conforming support to the rounded edge of the femoral runner. Because of its shape, location, and ability to flex and move somewhat as it is pushed, each meniscus helps support and stabilize the outer edge of a femoral runner, as the femoral runner presses, slides, and “articulates” against the portion of the tibial plateau beneath it.
However, because all four of the menisci inside a person's knees are in high-stress locations, and are subjected to frequently-repeated combinations of compression and tension (and sometimes abrasion as well, especially in people suffering from arthritis or other forms of cartilage damage), meniscal damage often occurs in the knees of humans, and occasionally other large animals.
It should be noted that, in humans, meniscal-type tissues also exist in temporomandibular, sternoclavicular, zygapophyseal, and wrist joints.
Various efforts have been made, using prior technology, to repair or replace damaged meniscal tissue. However, because of the complex structures and anchoring involved, and because of the need to create and sustain extremely smooth and constantly wet surfaces on the inner portions of each meniscal wedge, prior methods of replacing or repairing damaged meniscal are not entirely adequate.
Many meniscal implants for the knee address the need for attachment to the surrounding soft tissue but they do not address the need to resurface the femoral and/or the tibial articulating surfaces. An example of this type of implant is described by Kenny U.S. Pat. No. 4,344,193 and Stone U.S. Pat. No. 5,007,934.
A free-floating cobalt chrome meniscal replacement has been designed to cover the tibial bearing surface. Because this implant is rigid and because it is disconnected from the soft tissues it lacks the ability to shock absorb and/or provide proprioceptive input. In fact, because it is approximately 10–20 times more rigid than bone it may actually cause concentrated loading, increased contacts stresses, and therefore accelerate degenerative joint changes.
Various unicondylar knee implants for joint replacement contain a meniscus-like component. The tibial-bearing component of the known Oxford Knee (British Patent Application No. 49794/74) contains a free-floating piece of polyethylene that can glide or spin on a polished, flat, tibial CoCr surface in the transverse plane. The tibial-bearing component in turn articulates with the CoCr femoral implant. Because the polyethylene meniscus is semi-rigid it has a limited capacity to absorb shock or conform to the femoral component. Because of its materials, the Oxford knee also lacks a wear-reducing fluid film bearing surface.